Don’t try to do everything
==========================
A common temptation in Guild Wars is to try to do everything available to a
particular class.  This temptation is especially inherent in the Ranger class,
as Rangers are among the most versatile and widely-spread classes.  I can’t
remember how many times I’ve seen a Ranger who tries to squeeze Bow skills,
Traps, Stances, Preparations, Spirits, and a Pet into their skill bar.  While
it is fun early in the game to experiment with all sorts of skills, later in
the game such a character becomes increasingly spread thin.  Learn which skills
are the most useful to you, and specialize rather than diversify.  Find a niche
in your group and fill it.  Remember, you can always change your build at any
time when you are in town, so prepare the most effective combination for any
given mission, instead of doing everything.

Don’t overspecialize
====================
The opposite of the above mistake is the overspecialized ranger, one who
carries a full skillbar of Marksmanship skills (and no Res Signet, I might add)
while falling under an onslaught of Mountain Trolls because he/she forgot
Whirling Defense.  Generally this is only a problem with Marksmanship, although
I’ve seen folks who take their Traps way too seriously.  In general, most of
the Ranger skill lines are effective in 2-3 skill “clumps”, which you alternate
between in combat… a Ranger does NOT have enough Energy to continually use
bow attack skills, and the recharge times are quick enough on their skills to
make it unnecessary to bring more than 3. Always leave room for some healing or
defense and that 8th slot for your Res Signet or Signet of Capture.

Don’t drag along a Pet…
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… unless you are willing to be a full-time Beast Master.  Pets can be helpful
in early PvE game without many points in Beast Mastery, but as the game
progresses, you’ll find the Pet to be more and more of a liability unless you
start to invest heavily in Beast Mastery.  It’s almost an “all or nothing”
affair.  In order to have an effective pet, you have to treat it just like a
weapon, and an exclusive weapon at that.  If you aren’t willing to put most of
your points into Beast Mastery and devote 4 to 6 slots to your Pet, leave it
behind.  You can always decide to become a Beast Master at a later date.

This is not to say that there are no effective hybrid builds involving a Pet
and other themes.  But these sort of builds are difficult to tweak, and you are
too likely to be spread too thin (see “Don’t overspecialize” above) until you
have the attribute points and the skills to compensate for it later.  You can
ask yourself these questions:
* Do you have at least two Pet defensive skills (Call of Protection, Otyugh’s
  Cry, and/or Symbiotic Bond)?
* Do you have at least one Pet Attack?
* Is your Beast Mastery greater than 10?

If you said no to any of the above questions, you probably should leave the Pet.

You are not a tank
==================
While Rangers can be quite effective at preventing damage to themselves, you
are still vulnerable to many skills and attacks.  If you are in a group with
Warriors, let the Warriors do their job and tank the attacks.  Pound for pound,
the Warriors will take less damage (due to Damage reduction and better armor),
thus allowing healers to conserve more energy by casting healing spells less
often.  This doesn’t mean that you couldn’t occasionally draw some fire away
from the tanks (Throw Dirt is one of the best ways to do this), but unless no
one else can do this job, avoid charging in the middle of the fray to take the
brunt of the damage.  It is far more Energy-efficient for healers to aid the
slowly-falling health bars of the warriors than the soon-to-be-dead foolish
ranger.  Even with your defensive skills, you are going to take hits, and those
hits are going to hurt… it’s not the frequency of the hits that taxes the
average Monk healer, but the magnitude.

Don’t ignore Expertise
======================
At first glance, Expertise may look like a bum deal to a new Ranger. After all,
each point only reduces a small amount of Energy costs, and even then, it only
applies to Ranger skills for the most part. Expertise is the linked attribute
to some of the most powerful Ranger skills, like Whirling Defense, Lightning
Reflexes, and Throw Dirt.  With high enough Expertise, a Ranger should never
run into Energy problems.  A high Expertise greatly increases the staying power
of the Ranger, and most rangers should invest some attribute points into
Expertise.  My personal recommendation is a minimum Expertise of 8, when you
reach the level 20 endgame.  This is the breakpoint that reduces 5 Energy skills
down to 3 Energy, making those skills very easy to manage.

Nature Rituals are both friend and foe
======================================
One concept that new players have a hard time grasping is the fact that Nature
Rituals affect both allies and enemies in combat.  The trick to using Nature
Ritual spirits is to find a way to benefit the allies more than the enemies.
For example, Favorable Wind is a popular Spirit that increases arrow attack
speed and damage.  This Spirit is quite beneficial to Rangers (and other
Archers) when going up against opponents who lack bows and arrows.  However,
this Spirit also can mean the death of the party in areas where the enemy has
archers, such as the Skeleton Archers in Gates of Kryta or the Avicara Fierces
in the Shiverpeaks.  Leave the Favorable Winds spirit at home if you are going
up against archers.

There is nothing wrong with being “just” a Ranger
=================================================
As with all classes, there is nothing wrong with sinking all of your attribute
points into your primary class.  While half of the fun of making builds is
figuring out ways to combine the classes, often new players will start to sink
their points into too many areas and end up with a sub-par build.  Focus on the
basics of being a Ranger first, and you’ll be a lot more effective.  Note that
this doesn’t mean that you should ALWAYS put points into Ranger attributes.
But figure out the strengths and weaknesses of the Ranger first before you
start compounding the strengths and weaknesses of your secondary class as well.

When being hit by an AoE spell, MOVE!
=====================================
There are several AoE spells that target an area over a period of time. Most of
these are static area of effects, and can only deal sustained damage on you if
you stand still.  The simple solution, of course, is to move outside of the AoE
and reduce the damage done to yourself.  However, I see a lot of people who
just stand there and take it, oblivious to the damage numbers piling up.

Examples of this include Firestorm (a common Elementalist AoE), Chaos Storm (a
common Mesmer AoE), Ice Imps with Maelstrom, Mahgo Hydras with Meteor Shower,
and Sand Elementals with Eruption (which will blind you if you don’t move out
of the area of effect).